Catalytic reduction of emissions from internal combustion engines

ABSTRACT

An apparatus for treating an exhaust gas stream from cold startup through continuous operating conditions of an internal combustion engine includes an oxidizing catalyst bed disposed in an exhaust pipe and a reducing catalyst bed disposed in the exhaust pipe downstream from the oxidizing catalyst bed. The oxidizing catalyst bed as one or more oxidizing catalysts and the reducing catalyst bed has one or more reducing catalysts. A method is provided for treating an exhaust gas stream both during cold start and during continuous operating conditions of an internal combustion engine by passing the stream through an oxidizing catalyst bed having one or more oxidizing catalysts at a light off temperature; a reducing catalyst bed having one or more reducing catalysts and providing hydrogen into the reducing catalyst bed to condition the reducing catalyst; and introducing hydrogen into the internal combustion engine during cold startup.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for preventing,decomposing and removing emissions from an internal combustion engine.More particularly, the invention relates to a method and apparatus forpreventing, decomposing and removing emissions under both cold startconditions and during continuous operation of an internal combustionengine.

2. Background of the Related Art

The exhaust gases from boilers, smelters, diesel generators, jetengines, gas turbine engines, automobiles, and trucks containconsiderable amounts of nitrogen oxide compounds (NO_(x)), unburnedhydrocarbons (HCs) and carbon monoxide (CO). Nitrogen oxide, thoughthermodynamically unstable, does not spontaneously decompose in theabsence of a catalyst. For emissions from engine operations usuallynear-stoichiometric air/fuel ratios, the reduction of NO_(x) by CO andresidual hydrocarbons is achieved by what is often called a “three waycatalyst” (TWC). No satisfactory catalyst system exists, however, forNO_(x), HCs and CO abatement in exhaust gases from internal combustionengines which contain an excess of fuel under cold start conditions andan excess of oxygen under continuous operation conditions.

TWC catalysts are currently formulated and designed to be effective overa specific operating range of both lean and rich fuel/air conditions anda specific operating temperature range. These particulate catalystcompositions enable optimization of the conversion of HCs, CO, andNO_(x). This purification of the exhaust stream by the catalyticconverter is dependent upon the temperature of the exhaust gas and thecatalytic converter works optimally at an elevated catalyst temperature,generally at or above about 300° C. The time period between when theexhaust emissions begin (i.e., “cold start”), until the time when thesubstrate heats up to a light-off temperature, is generally referred toas the light-off time. Light-off temperature is generally defined as thecatalyst temperature at which fifty percent (50%) of the emissions fromthe engine are being converted as they pass through the catalyst.

The conventional method of heating the catalytic converter is to heatthe catalyst by contact with high temperature exhaust gases from theengine. This heating, in conjunction with the exothermic nature of theoxidation reactions occurring at the catalyst, will bring the catalystto light-off temperature. However, until the light-off temperature isreached, the exhaust gases pass through the catalytic converterrelatively unchanged. In addition, the composition of the engine exhaustgas changes as the engine temperature increase from a cold starttemperature to an operating temperature, and the typical TWC is designedto work best with the exhaust gas composition that is present at normalelevated engine operating temperatures.

Selective Catalytic Reduction (SCR) is one measure that is beingexplored with regard to NO_(x) reduction. Ammonia is injected into theexhaust gases to react with NO_(x) over a catalyst to form nitrogen andwater. Three types of catalysts have been used, including base metalsystems, noble metal systems and zeolite systems. The noble metalcatalysts operate in a low temperature regime (240-270° C.), but areinhibited by the presence of SO₂. The base metal catalysts, such asvanadium pentoxide and titanium dioxide, operate in the intermediatetemperature range (310°-400° C.), but at high temperatures they tend topromote oxidation of SO₂ to SO₃. The zeolites can withstand temperaturesup to 600° C. and, when impregnated with a base metal, have an evenwider range of operating temperatures.

SCR systems with ammonia as a reductant have been successfully employedto yield NO_(x) reduction efficiencies of more than 80% in large naturalgas fired turbine engines, and lean burn diesel engines (that run richin oxygen). However, problems arise due to a strong dependence of theammonia reaction and the catalyst life on exhaust gas temperature. Therequirement of ammonia itself presents several problems. Ammonia is atoxic gas and is included in the EPA's list of extremely hazardoussubstances. The most critical aspect of SCR systems include safe ammoniahandling, control of reactor temperature at all operating conditions,control of exhaust temperatures, and a dynamic ammonia dosage controlsystem to maintain an optimum ammonia/NO_(x) mole ratio under varyingengine speed and load conditions. Some ammonia slip is unavoidable dueto imperfect distribution of the reacting gases.

Selective Catalytic Reduction with hydrocarbons is another measure usedto reduce NOx emissions. NO_(x) can be selectively reduced by a varietyof organic compounds (e.g. alkanes, olefins, alcohols) over severalcatalysts under excess O₂ conditions. The injection of diesel ormethanol has been explored in heavy-duty stationary diesel engines tosupplement the HCs in the exhaust stream. However, the conversionefficiency was significantly reduced outside the narrow temperaturerange of 300° C. to 400° C. In addition, this technique suffers from thesame problems as those of SCR with ammonia, such as HC-slippage over thecatalyst, transportation and on-site bulk storage of hydrocarbons, andpossible accidental release of the HCs into the atmosphere. The partialoxidation of hydrocarbons also releases undesirable CO, unburned HCs andparticulates.

Another method to decrease NOx proposes using combustion at excessivelylean air-fuel ratios to provide a combination of a decrease in NOxemissions and an increase in fuel economy in a lean burn engine.However, when a vehicle engine is operated at air-fuel ratios leanenough to decrease NOx, the combustion approaches a misfire limit, anddriveability is impaired. To prevent this, an improvement has beenproposed, wherein turbulences are generated within an engine cylinder sothat the burning velocity is increased to thereby shift the misfirelimit to the lean side. However, if the turbulences are excessive andthe flow velocity becomes too high, formation of a flame core andpropagation of the flame in an early period of combustion will beobstructed. Another improvement has been proposed, where the air-fuelratio distribution within an engine cylinder is controlled so that richair-fuel mixtures are formed only in a region close to the ignition plugto produce easy ignition. However, when the misfire limit is shifted tothe lean side, the effect on the NOx concentration also is decreased.

Another proposed method to decrease NOx provides an engine with air-fuelratios slightly closer to the stoichiometric air-fuel ratio than themisfire limit and then purifies the insufficiently decreased NOx byusing a zeolite-type lean NOx catalyst. This method has the potential toprovide a clean system that also has good fuel economy. However, sincethe lean NOx catalyst can operate only under oxidizing exhaust gasconditions and is usually exposed to high temperatures, it is difficultto obtain both a sufficiently high NOx conversion by the lean NOxcatalyst and a durable catalyst.

Lean burn engines, including lean burn gasoline engines and dieselengines, produce an exhaust gas that has an excess of oxygen (O₂), thatis, they are operated under oxidizing gas conditions. The leaner theair-fuel ratio, the greater is the concentration of O₂ included in theexhaust gas. A catalyst which reduces NOx under oxidizing gas conditionsis defined as a lean NOx catalyst, which is usually composed of a noblemetal-type catalyst or a zeolite-type catalyst. At temperatures above350° C., NOx reduction occurs primary by reaction with HC, while at lowtemperatures below 250°-350° C., NOx reduction occurs primarily byreaction with hydrogen (H₂), wherein NOx purification by H₂ is possible.

However, since the lean NOx catalyst is usually installed in or near anengine exhaust manifold in a conventional exhaust system, thetemperature to which the catalyst is exposed is as high as 800°-900° C.Further, since the lean burn engine is operated at above stoichiometricair-fuel ratios, almost no H₂ remains in the exhaust gas. Therefore, theNOx reduction characteristic of a lean NOx catalyst at low temperaturesbelow 250°-350° C. has not been used in a conventional lean burngasoline engine or diesel engine.

Therefore, there is a need for a cost effective, fuel efficient methodand apparatus for decreasing NO_(x), HCs and CO emissions from internalcombustion engines. It would be desirable if the method and apparatusremoved HCs, CO and NOx during cold start as well as during continuousoperation of an internal combustion engine. It would be furtherdesirable if the method and apparatus could be implemented on existingengines and did not require large inventories of chemicals.

SUMMARY OF THE INVENTION

The present invention provides an emissions preventing apparatus for anexhaust pipe in communication with exhaust from an internal combustionengine. The apparatus has an oxidizing catalyst bed disposed in theexhaust pipe and a reducing catalyst bed disposed in the exhaust pipedownstream from the oxidizing catalyst bed. A source of hydrogen has afirst control valve providing fluid communication with the oxidizingcatalyst bed and a second control valve providing fluid communicationwith the reducing catalyst bed. A source of oxygen has a control valveproviding fluid communication with the oxidizing catalyst bed. A controlsystem is provided for conditioning the oxidizing catalyst bed prior toreceiving significant amounts of exhaust having a component selectedfrom HCs, CO or combinations thereof and conditioning the reducingcatalyst bed prior to receiving significant amounts of exhaust havingNOx.

Preferably, the oxidizing catalyst bed comprises a three way catalystthat is conditioned during a cold start ignition by opening the firsthydrogen control valve and the oxygen control valve. The reducingcatalyst bed is preferably conditioned by opening the second hydrogencontrol valve. The oxidizing catalyst bed is preferably conditioneduntil reaching a light off temperature while the reducing catalyst canbe conditioned continuously or discontinuously throughout operation ofthe internal combustion engine. Preferably, the reducing catalystmonolith includes essentially no catalysts capable of oxidizingnitrogen.

Preferably, hydrogen delivery ports are provided in communication withone or more regions of the reducing catalyst monolith. The hydrogensource preferably includes an on-board electrolyzer having an anode forproducing oxygen, wherein the anode is in fluid communication with theoxygen source. The hydrogen source may also be in fluid communicationwith the internal combustion engine.

In another embodiment of the present invention, there is provided amethod for treating the exhaust gas from an internal combustion engine.The method includes passing the exhaust gas over one or more oxidizingcatalysts and then over one or more reducing catalysts; oxidizing theexhaust gas over the oxidizing catalyst; providing hydrogen gas into thereducing catalysts; and reducing the exhaust gas over the reducingcatalysts. Preferably, the one or more reducing catalysts is selectedfrom Pt, Ru, Pt-alloys, Ru-alloys or combinations thereof.

Preferably, hydrogen is provided to the reducing catalysts and nitrogenoxides are reduced to nitrogen gas and water vapor at the reducingcatalysts. Typically, the exhaust gas comprises one or more oxidizablecomponents selected from hydrocarbons, carbon monoxide or combinationsthereof and one or more reducible components selected from nitrogenoxides, sulfur oxides or combinations thereof. In accordance with theinvention, the internal combustion engines can burn fuel selected fromgasoline, diesel, natural gas or methanol.

In a preferred embodiment, the hydrogen is provided to the reducingcatalysts only after an engine warm-up period. Hydrogen can besubstantially continuously provided to the reducing catalysts after theengine warm-up period or it can be pulsed. The hydrogen may also beprovided to the reducing catalysts before an engine warm-up period tocondition the reducing catalysts prior to introducing nitrogen oxide.Alternatively, the hydrogen is preferably provided to the reducingcatalysts before the exhaust gas stream contacts the reducing catalysts.Preferably, hydrogen and oxygen are provided to the oxidizing catalystsat a time selected from before the internal combustion engine is startedor before the exhaust gas stream contacts the first catalyst monolith.

In addition, the oxidizing catalysts may be heated by exothermiccatalytic combination of hydrogen and oxygen up to a light-offtemperature. After the engine warm-up period, the hydrogen may besubstantially continuously provided to the reducing catalysts or pulseddepending on the temperature of the reducing catalyst. If the reducingcatalyst is too hot, the hydrogen delivery may be interrupted until thetemperature drops to an acceptable level for optimal NOx reduction.

The hydrogen is preferably produced electrolytically at a rateproportional to the load on the internal combustion engine. Theelectrolyzer can be started and hydrogen provided to the reducingcatalysts only after an engine warm-up period. After the engine warm-upperiod the hydrogen is substantially continuously provided to thereducing catalysts or the hydrogen is discontinuously provided to thereducing catalysts. The oxidizing catalysts are preferably heated byexothermic catalytic combination of hydrogen and oxygen up to alight-off temperature.

A portion of the electrolytically produced hydrogen can be accumulatedin a hydrogen storage vessel. The hydrogen provided to the oxidizingcatalysts during cold start is preferably supplied from the hydrogenstorage vessel. Hydrogen delivery to the oxidizing catalysts ispreferably stopped after the oxidizing catalysts reach a light-offtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the above recited features and advantages of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference to theembodiments thereof which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic diagram of the apparatus of the present inventionfor catalytically reducing emissions from internal combustion engines;

FIG. 2 is an exploded view of an electrolyzer that may be employed inthe present invention;

FIG. 3 is a schematic of a hydrogen capturing and handling detail usedwith the system of the present invention.

FIGS. 4 and 5 are catalyst monoliths having a hydrogen distributor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for catalytically convertingHCs, CO, and NO_(x) in stationary and mobile sources such as boilers,smelters, diesel generators, jet engines, gas turbine engines,automobiles, and trucks under cold start and during continuous operatingconditions. More specifically, the process involves introducing hydrogengas and a source of oxygen gas, if needed, into an exhaust streamupstream of an oxidation catalyst to bring the oxidation catalyst to itslight off temperature quickly, thus enhancing the catalytic oxidation ofHCs and CO into C0 ₂ and H₂O. Simultaneously, hydrogen gas is introducedupstream of a reducing catalyst to catalytically reduce NO_(x) intoharmless H₂O and N₂, where the educing catalyst is located downstream ofthe oxidizing catalyst.

More specifically, the invention relates to a system for cleaning uplean burn exhaust that is applicable to a “lean burn engine” which usesa dilute air-fuel mixture in order to improve fuel economy, as well as adiesel engine, a hydrogen engine and a Stirling engine (anexternal-combustion engine) and which is capable of effectively reducingand cleaning up nitrogen oxides in the exhaust irrespective of itsconcentration of oxygen gas without impairing the good fuel economy ofthose engines.

One aspect of the present invention provides a system that removes HCs,CO and NO_(x) emitted from a stationary or mobile internal combustionengine under cold start and during continuous operating conditions byproviding temporally and spatially a hydrogen enriched oxidizingenvironment and a hydrogen enriched reducing environment. The systemincludes a reducing catalytic converter having at least one monoliththat is coated with a catalyst suited for reducing NO_(x) and anoxidizing catalytic converter having at least one monolith coated with acatalyst suitable for oxidizing HC and CO. Hydrogen gas is injected intothe exhaust pipe upstream of the oxidizing monolith prior to, during andafter start up to heat the catalyst to a light off temperature. A sourceof oxygen may also be provided to the oxidizing catalyst. Whenoxygen-rich fuel mixtures are used, no oxygen is needed for theoxidizing catalyst. The exhaust stream then enters the oxidizingmonolith where HCs and CO are oxidized and is passed to the reducingmonolith where hydrogen gas is introduced into the reducing monolith.The hydrogen is preferably generated on board the vehicle or otherequipment securing the engine. Continuous hydrogen production may bemaintained on board the vehicle or stationary engine with the use of anelectrolyzer, such as the proton exchange membrane (PEM) electrolyzerdescribed in U.S. patent application Ser. No. 08/682,024 ('024) which isincorporated by reference in its entirety herein.

In another aspect of the present invention, an electrolyzer is used toproduce hydrogen on demand, on board the vehicle or engine such that thecurrent applied to the electrolyzer is increased to increase hydrogenproduction as the load on the engine increases. Thus, as the amount ofexhaust produced increases, more current is applied to the electrolyzerto increase hydrogen production accordingly.

The ability to generate and store hydrogen on board and on demand avoidsmost user maintenance requirements of other systems. The hydrogengeneration, storage, and delivery system described in the '024application uses a proton exchange membrane (PEM) electrolyzer to splitwater into hydrogen and oxygen. As the hydrogen gas forms, it iscompressed by the electrolyzer with efficiencies approaching theoreticalvalues and without any moving or wearing components. It is not necessaryto draw power from a battery, e.g., a starting, lighting, and ignition(SLI) battery, since the electrolysis process, for the replacement ofhydrogen, preferably does not begin until the engine has reachedoperating temperature. The technical advantages of this system include,but are not limited to: (i) hydrogen is generated from a small amount ofwater; and (ii) there are no moving parts or wearing components. Theelectrolyzer system does not require any modifications to existingengines or vehicles and retrofitting can be cost-effectivelyaccomplished. The electrolyzer is scalable to vehicles or heavyequipment of any size, ranging from small generators to engines rated atthousands of kilowatts.

Another aspect of the present invention relates to decreasing HC, CO andNO_(x) emissions by injecting hydrogen and a source of oxygen, ifneeded, into an oxidizing catalytic converter prior to, during, andafter startup to heat the oxidizing catalyst to an optimum catalytictemperature and injecting hydrogen into a reducing catalytic converterto condition the reducing catalyst in the reducing catalytic converter.Conditioning of the catalyst as referred to herein includes heating,cleaning, and/or activating the catalyst as well as saturating thecatalyst and the catalyst support material with hydrogen. Since amuffler is usually disposed in a tail pipe of an exhaust conduit, theexhaust gas may be over cooled, particularly during very cold weatherconditions, before flowing into the reducing catalytic converter.

Conventional three-way catalysts and conventional Cu/zeolite-type leanNOx catalysts using HC to reduce NOx show activity in a temperaturerange above 300°-400° C. and are therefore not suitable for use in theexhaust pipe near the muffler. As discussed above, however, when NOxreduction is effected by H₂ instead of by HC, NOx purification at lowtemperatures is possible. The reducing catalyst can be maintained in atemperature range of from 100°-300° C., preferably between 120°-260° C.,and most preferably between 125°-200° C. It is important to note thatthe temperature of the reducing catalyst should not fall below about100° C., because water vapor in the exhaust stream will convert toliquid form and saturate the zeolite, alumina, silica or other supportfor the catalyst, thus reducing the effectiveness of the catalyst.

The present invention makes it possible to maintain the reducingcatalyst at low temperatures. With a conventional emission removalapparatus, it would be necessary to operate the catalyst at highertemperatures because HCs and CO could be oxidized off and thus would notbe allowed to stick to and eventually poison the reducing catalyst. Inthe present invention, HCs and CO are removed by the oxidizing catalystprior to, during, and after startup of the engine. The removal of theseHCs and CO prior to, during, and after startup of the engine greatlyincreases the efficiency of the reducing catalyst by allowing it to bemaintained at temperatures that are optimal for hydrogen initiatedreduction of NOx.

The reaction between NOx and hydrogen by nature produces heat, thereforeanother aspect of the present invention is directed to withdrawing theheat from the reducing catalytic converter. The reducing catalystmonolith is equipped with cooling fins and/or a typical tube-in-shellheat exchanger to remove the heat from the reducing catalyst. Themonolith would then have catalyst on the tube side and air flow on theshell side for cooling the monolith. The exhaust stream would passthrough the tube side of the heat exchanger and contact the catalyst asit passes through the monolith. Air from an auxiliary air pump may becooled and circulated through the heat exchanger but not into directcontact with the catalyst itself. Alternatively, suitable metalliccooling fins may be used to withdraw heat from the reducing catalyticconverter. A temperature sensor can be located in or near the reducingmonolith to monitor the temperature and initiate pulsed hydrogen flowwhen the monolith reaches a certain critical temperature. Reducing orrestricting the hydrogen flow temporarily will allow the monolith tocool and the NOx reduction will continue at least to some extent withresidual hydrogen present in the catalyst support material.

The lean burn engine and the diesel engine are basically the same inthat the exhaust contains excess O₂ and the concentration O₂ in theexhaust increases as the air/fuel ratio increases. The catalyst forreducing and cleaning up the NOx in such O₂ containing exhaust is calleda “lean catalyst”, which is often selected from among noble metal basedcatalysts such as those supported on zeolite. The reaction between HCand NOx occurs mainly in the high-temperature (>350° C.) range. On theother hand, in the low-temperature (<300° C.) range, a reaction forreducing NOx with H₂ occurs, enabling the cleanup of NOx. Another typeof catalyst for reducing and cleaning up the NOx in the O₂ containingexhaust is also called a “lean NOx catalyst”, which is selected fromamong those catalysts which support noble metals such as Pt. If CO oractive HC is present in gases that flow into the reducing catalyticmonolith, those gases will cover the surface of the catalyst throughadsorption, thereby blocking the reaction of NOx reduction with H₂.However, with the present invention, the HCs and CO are removed priorto, during, and after startup of the engine in the oxidizing catalyticconverter, such that the temperature of the reducing catalyst can bemaintained between 100°-300° C. with little concern for HC and COadsorption on the reducing catalyst in the reducing catalytic converter.

Conventionally, the catalyzer is installed near the exhaust manifold onthe engine, so the catalyst is exposed to the exhaust which is as hot as800°-900° C. at maximum. In addition, the exhaust from the lean burnengine which uses an air-fuel mixture leaner than the stoichiometricratio is substantially free from H₂. Under these circumstances, it hasbeen impossible in the prior art to utilize the characteristics of thecatalyst that is active at lower temperatures.

In contrast, the method and apparatus of the present invention have theadvantage that whether the engine is operated at an air/fuel ratioricher or leaner than the stoichiometric value or at the stoichiometricratio (i.e., irrespective of the presence or absence of O₂ from theexhaust or of the concentration of O₂ in the exhaust), NOx can bereduced with the catalyst so that the best performance of the engine andfuel can be selected without considering the conditions for reducing theNOx content.

Both the reducing and oxidizing catalytic converters described above mayhave more than one catalytic monolith segment within a single housing.Multiple monoliths will increase the amount of HC and CO oxidized aswell as the amount of NO_(x) reduced. Hydrogen can be introduced to allor some of the monolith segments in each catalytic converter.

On-board, on-demand electrochemically generated hydrogen greatlysimplifies and readily facilitates the use of hydrogen as a controlledchemical additive to exhaust streams leading to the removal of CO, HCand NO_(x) emissions. An automated water electrolyzer subsystem for theon-demand production of hydrogen can be easily integrated with variousengines, e.g., diesel generators. Such a water electrolyzer subsystemhas unique features that make it ideally suited for the proposedapplication. These include: (i) an instantaneous response time forinitiating hydrogen generation; (ii) excellent load followingcapabilities enabling a rapid response to changing engine operatingconditions; (iii) the capability of delivering pure hydrogen gas atpressures up to 1000 psi, if desired; and (iv) facilitates pressurizedstorage of hydrogen gas on-site, if needed, when carrying outmaintenance and repairs. The water electrolyzer can be fully automated,thus precluding the need for manual attention or operation.

The electrolyzer can be run by supplying a small portion of theelectrical energy generated by the combustion engine (diesel generator,automobile, gas turbine engine, etc.). It should also be noted thatstationary engines or other sources of exhaust gas may utilize othersources of electricity. The hydrogen gas generated by the electrolyzercan be fed directly to the flue gas or exhaust gas stream and passedover an oxidizing and/or reducing catalyst. The amount of hydrogensupplied can be regulated electronically in real time by simply varyingthe current applied to the electrolyzer.

A hydrogen storage vessel may be used to supply hydrogen to theoxidizing and reducing catalytic converters for cold start operations toincrease the temperature of the oxidizing catalytic converter asdescribed in application '024 and to condition the reducing catalyticconverter. The hydrogen is stored under pressure and is injected intothe oxidizing catalytic converter, along with air, if needed, preferablydelivered using a secondary air pump, to heat the oxidizing catalyst.Hydrogen may be fed from the storage vessel to the reducing catalystsimultaneously or after the engine warms up. The electrolyzer cancontinue to run, both to provide a sufficient stream of hydrogen to thereducing catalytic converter to reduce the NO_(x) emissions and toreplenish the stored hydrogen supply.

In addition, the stored hydrogen may be injected simultaneously into theengine as a substitute for fuel on start up. The hydrogen is preferablyinjected into the engine for up to one minute at start up, mostpreferably about 10-15 seconds, before the proscribed fuel is fed to theengine. Hydrogen addition to the oxidizing catalytic converter ispreferably begun at startup and continued until the catalyst reaches alight off temperature in accordance with the cold start proceduresoutlined above. Hydrogen is also delivered to the reducing catalyst tocondition the reducing catalyst before the NOx production becomessubstantial. This system can be adapted to work with engines that usegasoline, diesel, gasohol, methanol, natural gas or other fuels.

Hydrogen can be delivered intermittently to the reducing catalyst toincrease the amount of NOx reduced. A component which occludes nitrogenoxides is added to the catalyst for reducing nitrogen oxides while thefeeding of the reducing agent (H₂) is suspended. This helps increase theconcentration of the NOx in the catalyst. If the hydrogen is deliveredin pulses, the temperature of the reducing catalyst goes down, furtherincreasing the overall reduction of NOx. The component used to occludeNOx, can be selected from alkali metals, alkaline earth metals andmixtures thereof and loaded on the reducing catalyst.

Suitable reducing catalysts that are useful in hydrogen initiatedcatalytic reduction of NO_(x) include noble metal catalysts such as Pt,Ru, and metal alloys based on Pt and Ru. It is preferred that the noblemetal catalysts are highly distributed on a carrier having a highspecific surface area (greater than 100 m²/g) such as alumina, silica,or zeolite.

Other zeolite-based catalysts may be useful for reducing NOx such as Cubased ZSM-5 catalyst. ZSM-5 reduces NO_(x) under a wider range oftemperatures in a net oxidizing stream, even with water and SO₂ in thestream. Therefore, zeolite catalysts appear to be highly suitable forlean burn diesel and jet engines. In addition to Cu-ZSM-5, it isbelieved that Indium or Gallium based ZSM-5 catalysts may also be usefulcatalysts for NOx reduction.

It is believed that the Pt/TiO₂ catalyst for NO_(x) conversion may besuitable for gasoline engines, but may not be as effective indiesel-powered engines, which operate under excess air conditions,without a means for removing excess oxygen prior to reducing the NO_(x).However, Pt/TiO₂ and Ru/TiO₂ catalysts, along with the aforementionedcatalysts may be useful in any type of engine by adjusting theconcentration ratios of the catalyst to optimize the use of hydrogen asa reactant and reduce the likelihood that the hydrogen will be convertedto water as a result of combining with oxygen.

A cost-effective, safe, reliable and energy-efficient technology foron-site, on-demand generation of pure hydrogen is provided by theelectrolyzer described in application number '024. The electrolyzer,when supplied with electrical energy, splits water into hydrogen andoxygen. In a proton-exchange-membrane (PEM) electrolyzer, protons aretransported through the solid membrane electrolyte from the anode to thecathode. At the surface of the cathodic electrocatalyst, the protonsrecombine with electrons from an external circuit and are liberated ashydrogen gas molecules.

The lifetime of PEM water electrolyzers has been demonstrated to be inexcess of 14 years and projected to be over 30 years, depending on theoperating conditions. Water electrolyzers have been employed in severalapplications, especially, where reliability and purity of the gasesproduced are the primary concerns. The water electrolyzer disclosedherein is extremely effective in rapidly preheating the oxidizingcatalytic converter of gasoline powered vehicles or other engines. Theelectrochemically generated hydrogen and a source of secondary air ifrequired are introduced directly upstream of the oxidizing catalyticconverter and the converter is rapidly brought to operating temperatureby the catalytic combination reaction. With an air flow rate of 90liters per minute mixed with 11 vol % hydrogen, the front face of acatalyst-coated ceramic monolith reached 400° C. within two seconds and9% of the ceramic was heated to 400° C. in three seconds.

FIG. 1 shows a system 10 of the present invention installed on a vehicleexhaust system. The vehicle includes an oxidizing catalytic converter 11located in an exhaust line 42 from a vehicle's engine 15 exhaustmanifold, a reducing catalytic converter 31 in communication with theexhaust line 42 and an optional muffler 13 in the exhaust line 42 inbetween the oxidizing and reducing catalytic converters as shown. Theexhaust line 42 is provided with air, if required, from an air pump 44and hydrogen from a hydrogen inlet line 46. The air pump could be anysuitable air source, such as a receiver, for injecting air into theexhaust line at suitable pressure and volumetric flow rate to achievethe ideal air/hydrogen ratio mixture for heating the oxidizing catalystin the converter 11.

The hydrogen supply system of the invention generally includes a waterreservoir 48, an electrolyzer 50, and an optional hydrogen storagecylinder 52. As shown in FIG. 1, the electrolyzer 50 may preferablycompromise a plurality of stacked identical cells 51. The reservoir 48serves both as a water reservoir and as a separator for oxygen andwater. The reservoir 48 may be a vehicle's windshield washer fluidstorage container, but is preferably a dedicated separator allowingcollection and storage of oxygen via port 54. Water flows by gravitydrain or is pumped from the reservoir 48 to the electrolyzer 50 via adrain line 56. As the electrolyzer produces hydrogen and oxygen, theoxygen and entrained water flows naturally back to the reservoir 48 viaa return line 58.

The next major component of the hydrogen source is the electrolyzer 50,shown in greater detail in FIG. 2. In the following description of theelectrolyzer 50, the materials of construction referred to as“preferred” are the materials used in a test device to prove that theelectrolyzer 50 works for its intended purpose. In commercial productionmodels of the present invention, where possible, less expensivematerials will be used throughout, such as carbon steel for titaniumwhere possible, and plastic such as polypropylene where heat and stresswill permit the use of such material.

The electrolyzer 50 may be referred to herein as a proton exchangemembrane (PEM) electrolyzer 50. The proton exchange membrane itself mayprove corrosive in this environment in contact with certain substances,thus requiring the careful selection of the material of construction ofthe electrolyzer. For example, the PEM should only contact carbon orgraphite. However, those of skill in the art will readily recognizewhere less exotic materials than those listed in the followingdiscussion that are located away from the PEM material itself and theoxygen electrode catalyst can be readily employed without penalty. Forexample, graphite will be the material of choice in certain structuralelements, and not some obvious candidates such as copper, aluminum, oriron, which can corrode thus forming ions that can poison the oxygenand/or hydrogen electrode catalysts.

Now referring to FIG. 2, the PEM electrolyzer 50 is shown as a cellstack including a pair of endplates 60 and 62. The endplates 60 and 62are preferably titanium and measure 4.2″×4.2″×¼″. Adjacent the topendplate 60 is an anodic cell frame 64. The cell frame 64 is preferablya carbon fiber-filled TEFLON sheet, sold under the trademark ZYMAXX byDu Pont. The cell frame 64 retains a 1:1 molar ratio of iridium andruthenium dioxides (IrO₂/RuO₂) as the anodic electrocatalyst. The cellframe 64 also includes a plurality of flow ports 66 to permit the supplyof reactant (water) and/or removal of electrolysis product (oxygen gas).Below the cell frame 64 is an expanded titanium metal current collector(flow field) 68, preferably 25 Ti 40-3/32 from Exmet Corp. An anodesubstrate 70 is preferably a porous titanium plate measuring2.49″×2.49″×0.05″. Below the anode substrate 70 is a proton exchangemembrane 72, cut from a sheet of NAFION 117 from Du Pont which serves asa solid electrolyte material and which is 175 μm thick.

FIG. 2 depicts a gasket 74, one of perhaps several installed whererequired. Gaskets 74 are stamped from 0.033 inch thick fluorosiliconesheet (Viton) and from 0.005 inch thick unsintered PTFE sheet. Theelectrolyzer 50 further includes a cathode substrate 76 like the anodesubstrate 70 and an expanded titanium flow field.

Finally, the PEM electrolyzer 50 includes a cathodic cell frame 80formed of polychlorotrifluorethylene (PCTFE) sheet, sold under thetrademark KEL-F by Afton Plastics. The cathodic cell frame 80 retains afuel cell gas diffusion electrode containing high surface area colloidalplatinum, supported on platinum black, having platinum loading of 4.0mg/cm² as the cathodic electrocatalyst layer.

As shown in FIG. 2, the various components of the PEM electrolyzer arestacked together and retained with a plurality of tie rods 82,preferably 16 such tie rods. Stainless steel tubing, such as SS316, arethen screwed into four threaded ports on one of the titanium endplates.The ports are the water inlet port 56, the oxygen outlet port 58, and apair of hydrogen outlet ports 84. To minimize electrical contactresistance, the titanium endplates 60 and 62 and the expanded titaniummetal current collectors 68 and 78 may be electroplated with a thin filmof gold or other noble metals, such as platinum.

The cathode and the anode of the electrolyzer are of specialconstruction. The cathodic electrode structure for hydrogen evolution isfashioned from a commercially available fuel cell gas diffusion layer ona carbon cloth backing, which acts as a support for the activehydrophilic electrocatalyst layer. This active layer contains highsurface area colloidal platinum (100 m²/g), supported on carbon black(60 wt % Pt on C), yielding a platinum loading of 4.0 mg/cm². Thecathodic electrode structure, having an area of 40 cm², was hot-pressedonto one side of a segment of precleaned NAFION 117 PEM material.Hot-pressing was carried out between the plates of a hot-press elevatedto 200° C. for 60 seconds, and using a force of 15,000 pounds.

For the anodic electrocatalyst layer, a 1:1 molar ratio of iridium andruthenium chlorides are dissolved in ca. 8 ml of concentrated HCl andheated to almost dryness. The resulting chlorides are then dissolved inisopropanol to make an ink-line coating. A porous titanium plate, 0.05″in diameter from Astro Met of Cincinnati, Ohio, is etched in 12% HBF₄for 60 seconds and rinsed with isopropanol. This substrate is thencoated with the ink-like mixture and the solvent evaporated under lowheat of about 90° C. This coating and drying procedure is repeated seventimes, then the electrode is heated in a furnace at 400° C. for 10minutes in ambient air. The coating, drying, and furnace treatment isrepeated twice more, but with a final baking time of two hours insteadof 10 minutes.

Referring back to FIG. 1, the system further includes a hydrogen storagecylinder and various supporting components in addition to the reservoir48 and the electrolyzer 50, described above. The components include aliquid water trap 86 to eliminate most of the entrained water from thehydrogen exiting the electrolyzer, a solenoid valve 88 to blow out thetrap, a check valve 90, and a pressure relief valve 92 to protect thesystem against over pressurization. FIG. 3 depicts additional detailsand a preferred arrangement of the hydrogen gas handling and capturesystem.

As previously described, the electrolyzer 50 includes a proton exchangemembrane in its construction so that generated oxygen is vented to thewater source reservoir and the hydrogen generated can be accumulated atpressure. Prior to operation, the system of FIG. 3 permits purging withan inert gas, such as nitrogen. For safety reasons, all air is firstremoved from the system by attaching a nitrogen gas feedline at a purgegas inlet 94 downstream of a check valve 90. During the purgingoperation, the hydrogen storage cylinder or vessel 52, such as a metalhydride vessel, is detached at a quick disconnect 96. This operationeffectively seals both the vessel 52 and a gas line 98, to keep thepurge gas out of the vessel 52. The remainder of the system is thenpurged from the purge gas inlet 94 through a back pressure regulator100.

To charge the system with hydrogen, the needle valve 102 between thestorage vessel 52 and the back pressure regulator 100 is shut. Hydrogengas generated by the electrolyzer is processed through a four-stageprocess to remove entrained water (liquid or vapor) and any oxygencontaminant from the hydrogen stream before storage. The first stepinvolves removal of a small amount of entrained liquid water coming fromthe electrolyzer in the hydrogen gas. This entrained liquid water isremoved without a pressure loss by means of the entrained liquid watertrap 86. The second step involves cooling the hydrogen gas stream fromthe electrolyzer temperature to ambient in a condensing coil 104. Theelectrolyzer typically operates at about 20° C. above ambient, with theexact temperature depending on specific electrolyzer operatingconditions. This second step condenses a substantial portion of thewater vapor in the hydrogen gas stream. This condensed water couldabsorb a significant amount of alcohol, which may be present duringoperation using windshield washer fluid as the electrolyzer reactantfeed. The condensate is collected in a condensate collector 106 andremoved through a drain valve 108.

At this point, the hydrogen gas stream is still saturated with watervapor, but now at a lower temperature. This saturated gas stream is nextpassed into a zeolite-filled gas drier 110. This drier absorbs watervapor and any alcohol vapor present when using a windshield washer fluidfeed. Any oxygen contaminant present in the hydrogen gas stream is theneliminated in a catalytic recombiner or oxygen eliminator 112 to reduceit to water. Final clean-up of the hydrogen gas stream is accomplishedin a second zeolite absorber bed in a polishing drier 114. The polishingdrier removes traces of water produced by the catalytic recombiner 112.

The hydrogen gas handling system of FIG. 3 is designed for relativelyshort term operation; longer term operations, for example 100,000 miles,would utilize other methods of water removal known in the art. Asatisfactory metal hydride hydrogen storage unit is available fromHydrogen Consultants of Littleton, Colo. Such an available unit canstore 30 liters of hydrogen which can be delivered at 30-45 psig, withrecharging using hydrogen gas at 100-200 psig. More preferably, thehydrogen storage vessel is a pressure vessel made of a compositestructure, aluminum or ferrous-based alloys. A suitable hydrogen storagevessel of this type is available from Harless Specialties.

Referring back to FIG. 1, a power source 132 is coupled to a firsthydrogen solenoid valve 138, a second hydrogen solenoid valve 137 and athird hydrogen control valve 19 upon engaging the ignition switch 134.The third hydrogen control valve 19 provides fluid communication betweenthe hydrogen source and the internal combustion engine 15 through a flowline 17. The solenoid valve 137 may be opened when a thermocouple (notshown) indicates that the engine 15 has reached a certain temperaturewhere NOx emissions are likely to be produced. Alternatively, thesolenoid valve 137 may be opened at a predetermined period of time afterignition of the engine. In order to control the flow of hydrogen to theoxidizing catalyst, the solenoid valve 138 may remain open or be pulseduntil the thermocouple 136 reads a temperature equal to or greater thanthe light-off temperature. In another embodiment, the hydrogen outletfrom the electrolyzer may be provided in direct fluid communication withthe reducing catalytic converter instead of passing through the storagevessel 52.

Air from the air pump 42 may be delivered to a tube-in-shell heatexchanger in communication with the reducing catalyst in order to coolthe catalyst. Temperature sensor 51 can be located in the reducingmonolith 31 similar to thermocouple 136. When the temperature in thereducing monolith approaches a critical high temperature (above theoptimum working temperature set by the operator) air may be circulatedthrough the heat exchanger to cool the monolith. On the other hand, ifthe temperature falls below 100° C., air and/or hydrogen can beintroduced into contact with the reducing catalyst monolith to increasethe temperature of the monolith.

Also in FIG. 1, the electrolyzer 50 receives power from the source 132when the hydrogen pressure in or near the hydrogen storage vessel 52, asindicated by pressure sensor 133, falls below a setpoint pressurebetween about 50 psig and about 400 psig. It should be recognized thatthe power to the electrolyzer 50 is turned off when the pressure exceedsa high pressure setpoint, such as 400 psig. It should also be recognizedthat many other conditions may be considered in controlling theelectrolyzer.

FIGS. 4 and 5 are schematic views of two catalytic converters 189 havinghydrogen injection manifolds 190. The design of the catalytic convertersshown may be used for the reducing and the oxidizing catalyticconverters. In each of the figures, the converters 189 have multiplemonoliths 194 separated by a short distance for hydrogen introductionand diffusion. In FIG. 4, the manifold is external to the converter 189with a plurality of injection tubes 192 delivering hydrogen into thegaps 195. Conversely, in FIG. 5 the manifold is in the center of themonolith 194 with a plurality of holes for hydrogen delivery into thegaps 195.

1. An apparatus for treating exhaust from an internal combustion enginein communication with an exhaust pipe, comprising: an oxidizing catalystbed disposed in the exhaust pipe; a reducing catalyst bed disposed inthe exhaust pipe downstream from the oxidizing catalyst bed; a source ofhydrogen having a first control valve providing fluid communication withthe oxidizing catalyst bed, and a second control valve providing fluidcommunication with the reducing catalyst bed, and a third control valveproviding fluid communication with the internal combustion engine ; asource of oxygen having a control valve providing fluid communicationwith the oxidizing catalyst bed; and a control system for conditioningthe oxidizing catalyst bed prior to receiving significant amounts ofexhaust having a component selected from hydrocarbons, carbon monoxide,and combinations thereof, and conditioning the reducing catalyst bedprior to receiving significant amounts of exhaust having NO_(x), andproviding hydrogen to the internal combustion engine during cold start .2. The system of claim 1, wherein the oxidizing catalyst bed isconditioned during a cold start ignition by opening the first hydrogencontrol valve and the oxygen control valve.
 3. The system of claim 1,wherein the reducing catalyst bed is conditioned by opening the secondhydrogen control valve.
 4. The system of claim 2, wherein the oxidizingcatalyst bed is conditioned until reaching a light off temperature. 5.The system of claim 1, wherein the reducing catalyst is conditionedselectively continuous or discontinuous throughout operation of theinternal combustion engine.
 6. The system of claim 1, wherein theoxidizing catalyst bed is selected from a two-way catalyst and athree-way catalyst.
 7. The system of claim 1, further comprisinghydrogen delivery ports in communication with one or more regions of thereducing catalyst bed.
 8. The system of claim 1, wherein the reducingcatalyst bed includes essentially no catalyst for oxidizing nitrogen. 9.The system of claim 1, wherein the source of hydrogen includes anon-board electrolyzer.
 10. The system of claim 9, wherein the on-boardelectrolyzer has an anode for producing oxygen, and wherein the anode isin fluid communication with the oxygen source.
 11. The system of claim1, wherein the source of hydrogen further comprises a third controlvalve to provides hydrogen to the internal combustion engine during coldstart.
 12. A method for preventing and treating exhaust gas from aninternal combustion engine, comprising: supplying hydrogen fuel to aninternal combustion engine during cold start; providing a source ofhydrogen and a source of oxygen to the one or more oxidizing catalystsat a time selected from before the internal combustion engine is startedand before the exhaust gas stream contacts the one or more oxidizingcatalysts; passing the exhaust gas over one or more oxidizing catalystsand then over one or more reducing catalysts; oxidizing one or moreoxidizable components in the exhaust gas over the one or more oxidizingcatalysts; providing hydrogen gas to the one or more reducing catalysts,wherein the hydrogen is provided to the one or more reducing catalystsafter the engine warm-up period; and reducing one or more reduciblecomponents in the exhaust gas over the one or more reducing catalysts.13. The method of claim 12, wherein the one or more reducing catalystsare selected from Pt, Ru, Pt-alloys, Ru-alloys and combinations thereof.14. The method of claim 12, wherein the one or more reducible componentscomprises a nitrogen oxide, and wherein the nitrogen oxide is reduced tonitrogen gas and water vapor at the one or more reducing catalysts. 15.The method of claim 12, wherein the one or more oxidizable componentsare selected from hydrocarbons, carbon monoxide and combinations thereofand the one or more reducible components includes a nitrogen oxide. 16.The method of claim 12, wherein the internal combustion engine burns afuel selected from gasoline, diesel, natural gas and methanol after coldstartup.
 17. The method of claim 16, further comprising: providinghydrogen and oxygen to the one or more oxidizing catalysts at a timeselected from before the internal combustion engine is started andbefore the exhaust gas stream contacts the one or more oxidizingcatalysts.
 18. The method of claim 17 12, further comprising: heatingthe one or more oxidizing catalysts by exothermic catalytic combinationof hydrogen and oxygen up to a light-off temperature.
 19. The method ofclaim 17, wherein the hydrogen is substantially continuously provided12, further comprising providing hydrogen to the one or more reducingcatalysts after before the engine warm-up period.
 20. The method ofclaim 12, wherein the hydrogen is provided to the one or more reducingcatalysts only after an engine warm-up period.
 21. The method of claim20, wherein the hydrogen is substantially continuously provided to theone or more reducing catalysts after the engine warm-up period.
 22. Themethod of claim 12 19, wherein the hydrogen is provided to the one ormore reducing catalysts before an the engine warm-up period to conditionthe one or more reducing catalysts prior to introducing nitrogen oxides.23. The method of claim 12, further comprising electrolyticallyproducing the hydrogen at a rate proportional to the load on theinternal combustion engine.
 24. The method of claim 23, furthercomprising: storing a portion of the produced hydrogen in a hydrogenstorage vessel.
 25. The method of claim 24, wherein the hydrogenprovided to the one or more oxidizing catalysts is supplied from thehydrogen storage vessel.
 26. The method of claim 25, further comprising:stopping hydrogen to the one or more oxidizing catalysts after theoxidizing catalysts reach a light-off temperature.
 27. The method ofclaim 23, further comprising: starting the electrolyzer and providinghydrogen to the reducing catalysts only after an engine warm-up period.28. The method of claim 27 12, wherein the hydrogen is substantiallycontinuously provided to the one or more reducing catalysts after theengine warm-up period.
 29. The method of claim 27 12, wherein thehydrogen is discontinuously provided to the one or more reducingcatalysts after the engine warm-up period.
 30. The method of claim 12,further comprising: heating the one or more oxidizing catalysts byexothermic catalytic combination of hydrogen and oxygen up to alight-off temperature.
 31. The method of claim 12, further comprising:providing hydrogen to the one or more reducing catalysts before theexhaust gas stream contacts the one or more reducing catalysts.
 32. Themethod of claim 12 36, wherein the hydrogen is provided into theinternal combustion engine for about one minute or more followingstartup.
 33. The method of claim 12 36, wherein the hydrogen is providedinto the internal combustion engine for between about 30 seconds andabout one minute.
 34. The method of claim 12 36, wherein the hydrogen isprovided into the internal combustion engine for between about 10 andabout 15 seconds.
 35. The method of claim 12, wherein the one or morereducing catalysts are disposed on a support material selected fromalumina, silica, zeolite, and titanium dioxide.
 36. The method of claim12, further comprising supplying hydrogen fuel to an internal combustionengine during cold start.